Attempts have been made to enhance the turning or cornering stability of a lift truck by using stiff springs in castor wheels set at high pressure in order to make the truck more stable. But, use of stiff springs can also reduce the operator's comfort by allowing the operator to feel more bumps. Other stability enhancement mechanisms lock up the truck suspension in order to enhance truck stability when turning, but with similar results.
For example, referring to FIG. 1, a simplified front view of a lift truck 20 is shown using spring loaded casters 22 and a center drive tire 24 spaced between the two spring loaded casters 22. The spring loaded casters allow for lift truck turning and driving over rough floors 26 while still maintaining a smooth ride and good contact force for the drive tire 24. Typically, the casters 22 are adjusted to find an optimum operation between drive tire slippage and the truck rocking or tilting between both casters.
Other varieties of lift truck configurations use spring loaded casters and include a shock absorber 30 for damping. The addition of damping allows for softer springs, which reduces the rate of outward roll, but not the magnitude of the roll. Nevertheless, when the lift truck turns sharply, the damper responds to the high speed motion of the caster by generating a force and may still tilt the truck because the damper force is a function of caster motion, not truck roll. When the damper reacts in this way, it reduces operator comfort and diminishes the advantages of softer springs.
Referring to FIG. 2, still other varieties of lift truck configurations use fixed casters 34 and a suspended drive tire 36. This configuration lets the suspension spring 38 provide enough force to keep the suspended drive tire 36 in contact with the floor, and is more prevalent with very flat floors. Yet, on rough floors, operators of a truck with this configuration are known to feel oscillations and the truck may tilt on most every bump. Also, the effect of hitting an object with one caster may cause a noticeable contact and tilt felt by the operator.
At best, all of these previous configurations only improve the tradeoff between soft springs and the truck tilting (rolling or swaying while turning) versus hard springs, and truck dampers that limit the performance of the spring only caster configurations. If the spring tension of the caster could be adjusted during a turn, the truck would then be capable of turning without swaying and maintaining comfort for the operator.
What is needed are systems and methods for enhancing the steering performance by adjusting the caster spring tension in a turn, yet without sacrificing vehicle performance, cornering ability and/or operator comfort.